High strength - high ductility cold rolled recovery annealed steel and process for manufacture thereof

ABSTRACT

A high strength-high ductility cold rolled steel sheet is provided. The steel sheet has a recovery annealed microstructure, a yield strength greater than 820 megapascals (MPa) and a percent elongation to failure greater than 3.5%. In some instances, the steel alloy sheet has a Rockwell B hardness greater than 100 and may or may not exhibit a yield strength-to-tensile strength ratio between 0.25 and 1.00.

CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application claims priority of U.S. Provisional Application No. 61/791,145 filed on Mar. 15, 2013, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The use of cold rolling to increase strength levels of low alloy steel is known. However, cold rolling can also result in unacceptable low ductility levels of the material. As such, recrystallization annealing in a continuous annealing line (CAL) is known to be used to improve the ductility of cold rolled steels. But, recrystallization, annealing and the resulting increase in ductility typically results in a significant loss in yield and tensile strength of the material. Therefore, and in order to maintain desired yield and tensile strength levels, prior art steels have used significant addition of alloying elements with an associated increase in cost of material.

In addition to the above, the more expensive alloyed steels are batch annealed in order to perform recovery annealing and still maintain the strength levels. Such processing can produce strength levels greater than 820 megapascals (MPa) with ductility greater than 3.5% elongation for high alloy carbon steels. However, batch anneal processing takes a relatively long time compared to CAL processing and also results in temperature non-uniformity across the width of a batch annealed coil. Therefore, a significant variation in properties from edge to edge of the coil is known to be a problem.

Given the above, a more cost-effective alloy and efficient process to produce a cold rolled steel that has a strength of greater than 820 MPa and a ductility greater than 3.5% elongation would be desirable.

SUMMARY OF THE INVENTION

A high strength-high ductility cold rolled steel sheet is provided. The steel sheet includes a steel alloy having a chemical composition in weight percent within a range of 0.04-0.10 carbon (C), 1.0-1.65 manganese (Mn), 0.5 maximum (max) silicon (Si), 0.10 max chromium (Cr), 0.02-0.07 niobium (Nb), 0.03 max titanium (Ti), 0.003 max vanadium (V), 0.10 max molybdenum (Mo), 0.10 max nickel (Ni), 0.015 max sulfur (S), 0.025 max phosphorus (P), 0.012 max nitrogen (N), 0.001 max boron (B), and 0.015-0.065 aluminum (Al). The steel alloy sheet has a recovery annealed microstructure and a yield strength greater than 820 megapascals (MPa) and a percent elongation to failure greater than 3.5%. In some instances, the steel alloy sheet has a Rockwell B hardness greater than 100.

The high strength-high ductility cold rolled steel sheet with the recovery annealed microstructure has less than 10 volume percent (vol %) of recrystallized grains, preferably less than 5 vol % of recrystallized grains, and more preferably less than 2 vol % of recrystallized grains.

A process for making the high strength-high ductility cold rolled steel sheet is also included, the process including providing a steel alloy having the above-described composition and hot rolling the steel alloy to produce a hot rolled steel strip with a thickness of less than 10 millimeters (mm). In some instances, the hot rolled strip can have a thickness between 1.5 and 6.0 mm and is coiled at temperatures between 500 and 730° C., or in the alternative coiled at temperatures between 520 and 730° C. The process also includes cold rolling the hot rolled steel strip to produce a cold rolled steel sheet having a thickness that is less than 50% of the hot rolled steel strip thickness. Finally, the cold rolled steel sheet is recovery annealed such that the cold rolled steel sheet has a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%. In some instances, the steel alloy is hot rolled using a roughing treatment at temperatures between 1000 and 1200° C. and a finishing treatment having an entry temperature between 900 and 1100° C. and an exit temperature between 780 and 930° C.

In some instances, the cold rolled steel sheet has a thickness between 0.3 and 2.3 mm and may or may not be recovery annealed at temperatures between 580 and 660° C. The cold rolled sheet having been recovery annealed can be optionally temper rolled with between 0.0 and 3.5% deformation.

The steel alloy with the above-described chemical composition can be in the form of a steel slab with a thickness between 50 and 280 mm. The steel slab can be soaked at temperatures between 1150 and 1320° C. and subsequently hot rolled at temperatures between 780 and 1200° C. in order to produce a hot rolled sheet or strip with a thickness between 1.5 and 6.0 mm. The hot rolled strip can be coiled at temperatures between 520 and 680° C. and subsequently uncoiled and cold rolled to produce a cold rolled steel sheet having a thickness between 0.30 and 2.3 mm. The cold rolled steel sheet is recovery annealed in a continuous annealing line (CAL) at temperatures between 580 and 660° C. and may or may not be subjected to a temper rolling treatment. The final product has a yield strength greater than 820 MPa and a percent elongation greater than 3.5%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical plot illustrating as-rolled full hard yield strength versus percent reduction for soft steel and hard steel;

FIG. 2 is a graphical plot illustrating yield strength versus recovery anneal furnace temperature for samples subjected to 55%, 65%, and 70% reduction;

FIG. 3 is a graphical plot illustrating tensile strength versus recovery anneal furnace temperature for samples subjected to 55%, 65%, and 70% reduction;

FIG. 4 is a graphical plot illustrating elongation to fracture versus recovery anneal furnace temperature for samples subjected to 55%, 65%, and 70% reduction;

FIG. 5 is a graphical plot illustrating yield strength versus position—head (H), middle (M), tail (T)—on coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 6 is a graphical plot illustrating tensile strength versus position—head (H), middle (M), tail (T)—on coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 7 is a graphical plot illustrating percent elongation to fracture versus position—head (H), middle (M), tail (T)—on coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 8 is a graphical plot hardness (HRB) versus position—head (H), middle (M), tail (T)—on coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 9 is a graphical plot illustrating yield strength versus annealing temperature for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 10 is a graphical plot illustrating tensile strength versus annealing temperature for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 11 is a graphical plot illustrating percent elongation to failure versus annealing temperature for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 12 is a graphical plot illustrating hardness (HRB) versus annealing temperature for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 13 is a graphical plot illustrating yield strength versus percent skin pass degree for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 14 is a graphical plot illustrating tensile strength versus percent skin pass degree for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention;

FIG. 15 is a graphical plot illustrating percent elongation to fracture versus percent skin pass degree for coils of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention; and

FIG. 16 is a series of optical micrographs showing the microstructure of recovered but non-recrystallized grains for a sample taken from a coil of high strength-high ductility cold rolled recovery annealed steel produced according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A cold rolled low alloy steel sheet having a lower yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5% is provided. A process for producing such a cold rolled low alloy steel sheet is also provided. As such, the cold rolled low alloy steel and the process of manufacture have use for providing a material that can be used to make components, parts, etc.

The low alloy steel can be aluminum-killed and have a chemical composition in weight percent (wt %) within the range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.02-0.07 Nb, 0.03 max Ti, 0.003 max V, 0.10 max Mo, 0.10 Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, 0.015-0.065 Al and the remainder Fe. In some instances, the alloy composition has the above-stated or listed elemental ranges but with 0.10 max Nb and 0.10 max V. In addition, incidental impurities known to those skilled in the art with respect to production of steels can be present within the alloy.

A cold rolled low alloy steel sheet with the above-identified composition and properties is made by casting a slab with a desired composition and having a thickness between 50 and 280 mm. The slab is soaked at a temperature between 1150 and 1320° C. and then hot rolled during a roughing treatment at temperatures between 1000 and 1200° C. The roughing treatment produces a transfer bar having a thickness between 45 and 70 mm which is subjected to a finishing treatment where additional hot rolling is performed. The transfer bar enters the finishing treatment at an entry temperature between 900 and 1100° C. and exits the finishing treatment at a temperature between 780 and 930° C. Upon exiting the finishing treatment, the steel is in the form of hot rolled sheet which is formed or wound into a coil, i.e. coiled, at a temperature between 520 and 680° C. In addition, the hot rolled sheet has a thickness range between 1.5 and 6.0 mm.

The hot rolled sheet or strip is uncoiled and cold rolled with a reduction in sheet thickness ranging between 50 to 70%. After cold rolling, the cold rolled sheet is annealed in a CAL at a temperature or within a temperature range between 580 and 660° C. and a CAL speed between 30 and 200 meters per minute (m/min). The cold rolled sheet is then skin pass rolled or temper rolled with between 0.0-3.50% deformation. The above-described process produces a cold rolled sheet having a thickness between 0.30 and 2.3 mm that is recovery annealed and has a microstructure either void or at least partially void of recrystallization.

The recovery annealed cold rolled low alloy steel sheet has a lower yield strength greater than 820 MPa, a tensile strength greater than 820 MPa, a ductility measured by percent elongation at failure of greater than 3.5%, and a Rockwell B hardness greater than 100. In addition, the ratio of yield strength to tensile strength is between 0.25 and 1.0.

The process for producing the inventive material disclosed herein included using FIG. 1 to develop Equation 1 below. In particular, FIG. 1 is graphical plot of as-rolled full hard yield strength versus percent reduction for soft steel and hard steel and Equation 1 predicts as-cold rolled properties for steel alloys where Y_(CRFH) is the as-cold rolled full hard yield strength in MPa, Y_(HR) is the hot rolled yield strength in MPa, and R is the cold mill reduction to the material in percent.

Y _(CRFH)=196+Y _(HR)+4.2R  (Equation 1)

As shown by the expression, the final yield strength of as-cold rolled full hard steel sheet (Y_(CRFH)—without recovery annealing) can be determined from the yield strength of the starting hot rolled steel (Y_(HR)). Also, Equation 1 is limited to steel grades with a starting hot rolled strength between 350 to 600 MPa and subsequent percent cold reduction within the range of 30 to 90%.

Not being bound by theory, Equation 1 was used to determine the chemical composition or range of chemical compositions for a steel alloy having the desired mechanical properties listed above. Thereafter, three coils (Coils A, B and C) having desired chemical compositions were produced via hot rolling as described above, and then cold rolled to produce cold rolled sheet with a 55, 65 or 70% reduction in thickness. Samples/specimens from the cold rolled sheet where then subjected to a lab scale study in order to determine a desired recovery anneal temperature range.

The results of the lab scale study are shown in Table 1 below. Each specimen was held in a furnace for a period of time between 15-18 minutes as indicated below with the total time in the furnace calculated using the relation:

total furnace time=time to reach temperature+time in soak zone

where the time to reach temperature was calculated as the temperature divided by the heating rate of the furnace (40° C./min) and the soak zone calculated as the average CAL line speed (100 m/min) divided by the length of the CAL soak zone (100 m). The Coil A, Coil B and Coil C hot rolled strip specimens had initial thicknesses of 2.60 mm, 3.50 mm and 4.00 mm, respectively. In addition, all of the specimens were cold reduced to a final thickness of 1.20 mm in order to obtain the 55%, 65%, and 70% cold reduction as listed in the table. The cold-rolled specimens were then heat treated for the specified time and temperature as per Table 1 below, and then allowed to air cool. Tensile samples having a standard 50 mm gauge length per ASTM A-370-10 were machined from the recovery annealed specimens and used for tensile testing.

TABLE 1 SPECIMEN TEMPERATURE FURNACE TIME ID REDUCTION (%) (° C.) (min) COIL A A1 55 560 15 A2 55 600 16 A3 55 630 17 A4 55 660 18 COIL B B1 65 560 15 B2 65 600 16 B3 65 630 17 B4 65 660 18 COIL C C1 70 560 15 C2 70 600 16 C3 70 630 17 C4 70 660 18

The effects of percent reduction and furnace recovery anneal temperature on mechanical properties of the specimens are shown FIGS. 2-4. As shown by the figures, the recovery anneal temperature plays a role in the final mechanical properties for the material. In particular, FIGS. 2 and 3 illustrate a sharp reduction in yield strength and tensile strength, respectively, for a recovery anneal temperature of 660° C. Conversely, FIG. 4 illustrates the large increase in ductility observed for the recovery anneal temperature of 660° C.

Given the above results and data, parameters of 55±5% cold reduction and recovery annealing temperatures between 580-640° C. were set or established for recovery annealing in the CAL which are discussed below.

Two coils were produced from two separate slabs having thicknesses of approximately 250 mm and a composition within the range given above. A first slab was soaked at 1285° C. and then subjected to a roughing treatment. The roughing treatment provided a transfer bar having a thickness of 48 mm which was then subjected to a finishing treatment. The temperature at the entry of the finishing treatment for the first slab was 1075° C. and the temperature at the exit of the finishing treatment was 900° C. The hot strip had a thickness between 1.5 and 6.0 mm and was coiled at 550° C. The hot strip coil was then cold rolled 58% and recovery annealed at 640° C. with a CAL speed of 88 m/min.

The cold rolled and recovery annealed sheet from the first slab had a thickness of 1.20 mm, a lower yield strength of 870 MPa, a tensile strength of 900 MPa, a percent elongation to failure of 11.0%, and a Rockwell B hardness of 104.

The second slab was soaked at 1257° C. and then subjected to a roughing treatment to produce a transfer bar. The transfer bar entry temperature for the finishing treatment was 1050° C. while the exit temperature for the finishing treatment was 900° C. The hot rolled strip was then coiled at 590° C. and subsequently cold rolled 56% to produce a cold rolled sheet having a thickness of 1.6 mm. Thereafter, the cold rolled sheet was recovery annealed in a CAL at a temperature of 605° C. and a line speed of 80 m/min.

The cold rolled and recovery annealed mild carbon steel sheet had a lower yield strength of 868 MPa, a tensile strength of 890 MPa, a ductility of 9%, and a hardness of 103 HRB.

In order to test the variation of properties at different locations within coils having a chemical composition within the range provided above and produced according to a process taught above, the yield strength, tensile strength, elongation, and hardness were measured for samples taken from the head (H), middle (M), and tail (T) of a number of coils. The results of the testing for yield strength, tensile strength, elongation, and hardness are shown in FIGS. 5-8, respectively, with data in the figures illustrating that material from the various locations of the coils provided properties that meet or exceed the requirements of the material discussed above, i.e. lower yield strength greater than 820 MPa, tensile strength greater than 820 MPa, percent elongation greater than 3.5%, and hardness greater than 100 HRB. In some instances, properties of the coils greatly exceeded the requirements discussed above.

Looking at FIGS. 9-12, the yield strength, tensile strength, elongation, and hardness as a function of annealing temperature (labeled Strip temperature in the figures) for a plurality of processes according the parameters discussed above are shown. Again, the coils exhibited properties that met or exceeded the desired requirements discussed above.

FIGS. 13-15 provide data for yield strength, tensile strength, and percent elongation, respectively, as a function of skin pass degree. In particular, FIG. 15 illustrates the importance of skin pass or temper rolling on increasing the ductility of the steel sheet with a minimal decrease in yield and tensile strength (FIGS. 13-14). As such, it is appreciated and hereto for unknown in the prior art that skin pass/temper rolling can be used to increase the ductility of cold rolled recovery annealed steels. In the instant disclosure, a skin pass reduction of 0.5% resulted in an increase in ductility as measured by elongation to fracture from 4-6% to 8.8-12.8%. On average there was a two-fold increase in ductility. It is also appreciated that such ductility values allow for the steel compositions/grades to be used in a variety of industries and applications such as automotive applications, e.g. bumper beam support members. In addition, amount of degree of skin pass/temper rolling reduction is between 0.0 to 3.5%, preferably between 0.02 to 2.0%, more preferably between 0.02 to 1.0%, and still more preferably between 0.02 to 0.75%.

Finally, FIG. 16 provides a series of optical micrographs that show the microstructure of recovered but non-recrystallized grains for a sample taken from a coil having the chemical composition detailed above and subjected to the process described above.

It should be appreciated that the properties demonstrated by the inventive material and process disclosed herein are heretofore unknown in the prior art for such a low alloy steel and thus exhibit unexpected results. In addition, and in view of the teaching presented herein, it is to be understood that numerous modifications and variations of the present invention will be readily apparent to those of skill in the art. The foregoing is illustrative of specific embodiments of the invention, but is not meant to be a limitation upon the practice thereof. As such, the specification should be interpreted broadly. 

We claim:
 1. A process for making a high strength steel comprising: providing a steel alloy having a chemical composition in weight percent within a range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.02-0.07 Nb, 0.03 max Ti, 0.003 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, and 0.015-0.065 Al; hot rolling the steel alloy to produce a hot rolled steel strip having a thickness of less than 10 mm; cold rolling the hot rolled steel strip to produce a cold rolled steel sheet having a thickness less than 50% of the hot rolled steel strip thickness; and recovery annealing the cold rolled steel sheet, the recovery annealed cold rolled steel sheet having a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%.
 2. The process of claim 1, wherein the steel alloy is hot rolled using a roughing treatment at temperatures between 1000 and 1200° C. and a finishing treatment having an entry temperature between 900 and 1100° C. and an exit temperature between 780 and 930° C.
 3. The process of claim 2, wherein the hot rolled strip has a thickness between 1.5 and 6.0 mm.
 4. The process of claim 3, further including coiling of the hot rolled strip at temperatures between 500 and 730° C.
 5. The process of claim 4, wherein the cold rolled steel sheet has a thickness between 0.3 and 2.3 mm.
 6. The process of claim 5, wherein the cold rolled sheet is recovery annealed at temperatures between 580 and 660° C.
 7. The process of claim 6, further including temper rolling the recovery annealed cold rolled sheet with between 0.0 and 3.50 percent deformation.
 8. The process of claim 7, wherein the cold rolled sheet is tempered rolled between 0.02 and 2.0 percent deformation and the recovery annealed cold rolled sheet has a percent elongation to failure greater than 8.0%.
 9. The process of claim 8, wherein the cold rolled sheet is tempered rolled between 0.02 and 1.0 percent deformation.
 10. The process of claim 9, wherein the microstructure of the cold rolled steel sheet has less than 5 volume percent recrystallized grains.
 11. A process for making a high strength steel sheet comprising: providing a steel slab with a thickness between 50 and 280 mm, the steel slab having a chemical composition in weight percent within a range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.10 max Nb, 0.03 max Ti, 0.10 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, and 0.015-0.065 Al; soaking the steel slab at temperatures between 1150 and 1320° C.; hot rolling the steel slab at temperatures between 780 and 1200° C. and producing a hot rolled sheet with a thickness between 1.5 and 6.0 mm; coiling the hot rolled sheet at temperatures between 500 and 730° C.; cold rolling the hot rolled steel strip to produce a cold rolled steel sheet having a thickness between 0.30 and 2.3 mm; and recovery annealing the cold rolled steel sheet at temperatures between 580 and 660° C., the recovery annealed cold rolled steel sheet having a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%.
 12. The process of claim 11, wherein the hot rolling includes hot rolling the steel slab at temperatures between 1000 and 1200° C. and producing a transfer bar with a thickness between 45 and 70 mm.
 13. The process of claim 12, further including hot rolling the transfer bar at temperatures between 780 and 1100° C. and producing said hot rolled sheet with said thickness between 1.5 and 6.0 mm.
 14. The process of claim 13, wherein the cold rolled steel sheet has a microstructure with less 10 volume percent recrystallized grains.
 15. A high strength-high ductility cold rolled steel sheet comprising: a steel alloy sheet having a chemical composition in weight percent within a range of 0.04-0.10 C, 1.0-1.65 Mn, 0.5 max Si, 0.10 max Cr, 0.10 max Nb, 0.03 max Ti, 0.10 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, and 0.015-0.065 Al; said steel alloy sheet having a recovery annealed microstructure, a yield strength greater than 820 MPa and a percent elongation to failure greater than 3.5%.
 16. The high strength-high ductility cold rolled steel of claim 15, wherein said steel alloy sheet has Rockwell B hardness greater than
 100. 17. The high strength-high ductility cold rolled steel of claim 16, wherein said steel alloy exhibits a yield strength-to-tensile strength ratio between 0.25 and 1.00.
 18. The high strength-high ductility cold rolled steel of claim 17, wherein said recovery annealed microstructure has less than 10 volume percent of recrystallized grains.
 19. The high strength-high ductility cold rolled steel of claim 18, wherein said recovery annealed microstructure has less than 5 volume percent of recrystallized grains.
 20. The high strength-high ductility cold rolled steel of claim 19, wherein said recovery annealed microstructure has less than 2 volume percent of recrystallized grains. 